1. Field of Invention
This invention pertains generally to aircraft propulsion systems, and more particularly to an aircraft cyclic pitch control system including proprotor hub and drive yoke mechanism. With greatest particularity, the invention pertains to a cyclic pitch control mechanism which utilizes a torsion spring and associated linkages to transmit cyclic rotor control moments resulting from proprotor blade flapping to the airframe.
2. Description of the Related Art
Various types of aerial vehicles have been developed which are capable of vertical takeoff and landing, yet which also fly like a fixed wing aircraft. Some of these aerial vehicles are not intended to carry human operators or passengers. Such an aerial vehicle is termed an "Unmanned Aerial Vehicle" or UAV. The propelling blades are termed proprotor blades since they act both as rotor blades in helicopter flight mode and as propellers in fixed wing aircraft flight mode.
The present invention pertains to structure used in helicopter mode flight but that is also critical 1o performing the transition between helicopter and airplane flight modes. The term "proprotor blades" will be used to describe the propelling blades since they serve dual functions in different flight modes. The present invention is for use in such an aerial vehicle as described in U.S. patent application Ser. No. 08/206,558 which was filed Mar. 4, 1994, for VERTICALLY LAUNCHABLE AND RECOVERABLE WINGED AIRCRAFT by Nicholas Albion.
The UAV typically has a collective and cyclic pitch control system which allows the UAV to not only take off and land like a helicopter, but also to maneuver sideways in flight like a helicopter, and to adjust pitch while operating in the fixed wing aircraft flight mode. For flight in the fixed wing aircraft mode, the entire aircraft fuselage rotates from a vertical orientation to a horizontal orientation.
Cyclic pitch control refers to helicopter mode flight and means that proprotor blades may be positioned at different pitch angles during a portion of each proprotor blade revolution. Changing pitch during a single revolution usually means also changing angle of attack and consequently changing the lifting force produced by the proprotor blade during each revolution. This changing lifting force is generally referred to as proprotor blade flapping force. Such changing lift or flapping force gives rise to changing forces at the rotor hub which are used to control the motion and attitude of the entire aircraft.
For flight in the helicopter mode, the aircraft cyclic pitch control mechanism must incorporate means to transmit forces and moments from the proprotor hub flapping to the airframe in order to control aircraft flight. Conventional means for absorbing proprotor blade flapping forces use packs of coil springs or elastomeric bearings. These approaches typically are very heavy, bulky, expensive, and difficult to inspect. Coil springs have a nonlinear spring rate which adds to the difficulty. These mechanisms add to the aircraft's weight and complexity, thereby increasing thrust, power and maintenance requirements, and aircraft risk.
It is desirable to have an unmanned VTOL aircraft, capable of both normal fixed wing aircraft and helicopter flight, which is simple in design, mechanically reliable, relatively light, and efficiently maneuverable in both fixed wing aircraft and helicopter mode flight. It is further desirable to eliminate complicated mechanisms in the aircraft such as those necessary for absorbing proprotor blade flapping forces, which increase aircraft weight, power and maintenance requirements, and risk.